Hum noise caused by an electromagnetic wave etc. generated as harsh noise in an AC power supply is known. The hum noise is caused by an electromagnetic wave generated by an amplitude such as an AC power supply etc., and is composed of waves having a fundamental frequency having power substantially constant in time and a plurality of harmonic components. The hum noise is caused by a trouble in a circuit of an amplifier, an A/D (analog-to-digital) converter, a D/A (digital-to-analog) converter, etc. For example, when the insulation between the circuit and the power circuit is insufficient, the AC component of 50 Hz and 60 Hz as the frequency of commercial power incidentally influences a signal line, thereby causing the hum noise.
To remove the hum noise, there is a noise removal device proposed for removing noise by, for example, selecting and composing the spectrum relating to the noise frequency from the spectrum data of an audio signal converted into a frequency area segmented for each frame, and the amplitude data of each spectrum, and subtracting the composed noise waveform from an audio waveform, thereby removing noise.
However, a conventional device is not capable of detecting noise other than that having a specified frequency. For example, when the frequency of a clock signal provided for an A/D converter is shifted due to the trouble of a crystal oscillator and a frequency dividing circuit, the frequency of hum noise to be detected is also shifted, and is not allowed to correctly detect the hum noise.
When there is a stationary frequency component such as a chord etc., and if the stationary frequency component incidentally is fallen into a specified band for removing hum noise, then it may be misdetected as hum noise.
Furthermore, for example, the passband of a phone voice is 300 Hz through 3400 Hz, the power of the fundamental frequency band of the hum noise of 50 Hz through 60 Hz is attenuated, and the fundamental frequency of the hum noise may not correctly detected in the existing fundamental frequency estimating method using an auto-correlation etc.